The state of stress on plane AB is to be computed. To do this, the orientation of plane AB relative to the planes of the stress element must be established.

The angle θ is defined as the angle between the x face of the stress element and plane AB.

To find θ, we use the fact that a plane surface is defined by its normal. We'll construct the normals to both the x face and plane AB, and then we'll determine the angle between these two normals.